many older adults are less steady when performing submaximal contractions. This impairment is evident as greater force fluctuations during isometric contractions and enhanced acceleration fluctuations during slow, anisometric contractions. When expressed in the frequency domain, the force fluctuations are of significant amplitude. The Principal Investigator's studies suggest that the decrease in steadiness is not due to an increase in motor unit force or motor unit synchronization, but that it is mainly due to a decrease in the regularity of the discharge of motor unit action potentials. They hypothesize that the age-related decline in steadiness is due to an increase in the variability of the discharge rate of motor units. They now propose four aims to test this hypothesis. Steadiness will be quantified as the coefficient of variation of force during isometric contractions and as the standard deviation of acceleration during anisometric contractions. Discharge rate variability will be expressed as the coefficient of variation of motor unit discharge rate. Aim 1 evaluates directly the effect of discharge rate variability on steadiness during simulated isometric contractions. Aim 2 extends on previous work to examine the range of forces and loads over which there is an association between steadiness and discharge rate variability. Aim 3 proposes to determine if the improvements that occur in steadiness with strength training are accompanied by reductions in discharge rate variability. Aim 4 compares changes in steadiness and antagonist muscle coactivation, which is the only other variation in motor output that might influence steadiness. The experiments (Aims 2-4) will be performed on young and old adults as they perform isometric and anisometric contractions with the first dorsal interosseus muscle. The computer simulations (Aim 1) will be based on a model that they have been developing to examine the effects of motor unit synchronization on the EMG and force of steady-state contractions. They expect to find that discharge rate variability can cause decreases in steadiness (Aim 1), that the association between steadiness and discharge rate variability extends up to forces and loads less than 20% of maximum (Aim 2), and that training-induced improvement in steadiness is accompanied by a decline in discharge rate variability (Aim 3), and that coactivation of the antagonist muscle does not co-vary with steadiness (Aim 4). It is anticipated that these findings will suggest that differences in steadiness among individuals during submaximal contractions are due to differences in the variability of motor unit discharge rate.